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How can I solve Singelton is NULL problem [duplicate]

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Closed 1 year ago.
I'm trying to make a student registration system and I keep these students in firestore. I don't want it to add student with same number when adding student and for this, I created a singleton class. This singleton class has a flag value. I used this flag to provide control if there is a student with the same number in the firestore.
but it always returns null. I don't understand.
I'm just sharing the necessary codes.
My add student class
private ActivityOgrenciEkleBinding binding;
private FirebaseFirestore mFirestore = FirebaseFirestore.getInstance();
private LinkedHashMap<String,String> linkedHashMap;
private OgrenciyiKontrolEt ogrenciyiKontrolEt;
Singleton singleton;
public void init(){
linkedHashMap = new LinkedHashMap<>();
singleton = Singleton.getInstance();
btn_Ogrenci_EKLE();
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
binding = ActivityOgrenciEkleBinding.inflate(getLayoutInflater());
View view = binding.getRoot();
setContentView(view);
init();
}
public void btn_Ogrenci_EKLE(){
binding.btnEkleOgrenci.setOnClickListener(new View.OnClickListener()
{
#Override
public void onClick(View view) {
String email_ogr = binding.emailOgrenci.getText().toString();
String ad_ogr = binding.isimOgrenci.getText().toString();
String soyisim_ogr = binding.soyisimOgrenci.getText().toString();
String no_ogr = binding.numaraOgrenci.getText().toString();
String parola_ogr = binding.sifreOgrenci.getText().toString();
ogrenciyiKontrolEt = new
OgrenciyiKontrolEt(no_ogr,OgrenciEkleActivity.this);// I
//created object from
//control class to check students
if(email_ogr.equals("") || ad_ogr.equals("") || soyisim_ogr.equals("") || no_ogr.equals("") || parola_ogr.equals("")){
AlertDialog.Builder builder = new AlertDialog.Builder(OgrenciEkleActivity.this);
builder.setTitle("UYARI !");
builder.setMessage("Boş alan bırakmayınız !");
builder.setIcon(R.drawable.warningicon);
builder.setPositiveButton("Tamam", new DialogInterface.OnClickListener() {
#Override
public void onClick(DialogInterface dialogInterface, int i) {
}
});
builder.show();
}
else{
ogrenciyiKontrolEt.OGR_KONTROL();
System.out.println("singleton get flag"+singleton.getflag()); //singleton returns null here and and it never
//goes into the if loop
if("100".equals(singleton.getflag())){
AlertDialog.Builder builder = new AlertDialog.Builder(OgrenciEkleActivity.this);
builder.setIcon(R.drawable.warningicon);
builder.setMessage(no_ogr+" numaralı öğrenci zaten kayıtlı lütfen farklı bir numara giriniz.");
builder.setTitle("UYARI");
builder.setPositiveButton("TAMAM", new DialogInterface.OnClickListener() {
#Override
public void onClick(DialogInterface dialogInterface, int i) {
}
});
builder.show();
}
else{
linkedHashMap.put("name",ad_ogr);
linkedHashMap.put("lastname",soyisim_ogr);
linkedHashMap.put("number",no_ogr);
linkedHashMap.put("email",email_ogr);
linkedHashMap.put("password",parola_ogr);
mFirestore.collection("Students").add(linkedHashMap).addOnSuccessListener(new OnSuccessListener<DocumentReference>() {
#Override
public void onSuccess(#NonNull DocumentReference documentReference) {
Toast toast = Toast.makeText(OgrenciEkleActivity.this,"Öğrenci başaralı bir şekilde eklendi.",Toast.LENGTH_LONG);
toast.show();
}
}).addOnFailureListener(new OnFailureListener() {
#Override
public void onFailure(#NonNull Exception e) {
Toast toast = Toast.makeText(OgrenciEkleActivity.this,"Öğrenci eklerken bir hata ile karşılaşıldı.",Toast.LENGTH_LONG);
toast.show();
}
});
}
}
}
});
}
Control class that I created to control students
public class OgrenciyiKontrolEt {
protected String ogrenci_no;
protected String firestore_ogr_no;
protected FirebaseFirestore firebaseFirestoreDb = FirebaseFirestore.getInstance();
public Context context;
Singleton singleton = Singleton.getInstance();
public OgrenciyiKontrolEt(String ogr_no,Context context){
this.ogrenci_no = ogr_no;
this.context = context;
}
public void OGR_KONTROL(){
firebaseFirestoreDb.collection("Students")
.get()
.addOnSuccessListener(new OnSuccessListener<QuerySnapshot>() {
#Override
public void onSuccess(#NonNull QuerySnapshot queryDocumentSnapshots) {
List<DocumentSnapshot> snapshotList = queryDocumentSnapshots.getDocuments();
for(DocumentSnapshot snapshot: snapshotList ) {
firestore_ogr_no = snapshot.getString("number");
if(firestore_ogr_no.equals(ogrenci_no)) {
singleton.setflag("100"); //If there is a student with the same
number, I set the flag in the singleton to 100.
}
}
}
})
.addOnFailureListener(new OnFailureListener() {
#Override
public void onFailure(#NonNull Exception e) {
Toast toast = Toast.makeText(context,"Bir hata ile karşılaşıldı.",Toast.LENGTH_LONG);
toast.show();
}
});
}
}
My singleton class
public class Singleton {
**EDIT**
private String flag="1";
**EDIT**
private static Singleton singleton;
private Singleton() {
}
public String getflag() {
return flag;
}
public void setflag(String flag) {
this.flag = flag;
}
public static Singleton getInstance() {
if (singleton == null) {
singleton = new Singleton();
}
return singleton;
}
}

Your flag is not initialized when you call it, Either set something as default value or initialize in constructor like
private Singleton() {
this.flag = "";
}
or call your setFlag before first occurrence of getFalg

Interstitial Ad returns a null value

I programmed an app named "Trumple Run" for android. Like the name says it is a Jump & Run game and I want to release it soon. Before that I would like to implement Interstitial Ads from AdMob. I followed the official guide for the implementation of Ads but the app always crashes when I tried to show the add. Here is some code from my MainActivity (info: I cut out code which is not relevant for the issue; the method addzeigen() is called from another class)
public class Main_activity extends Activity {
private InterstitialAd mInterstitial;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN); //Fullscreen
DisplayMetrics metrics = getResources().getDisplayMetrics(); //Display größe herausfinden
V.Bildbreite = metrics.widthPixels;
V.Bildhöhe = metrics.heightPixels;
addladen();
V.v = new View(this);
setContentView(V.v);
}
public void addladen(){
mInterstitial = new InterstitialAd(this);
mInterstitial.setAdUnitId("ca-app-pub-3940256099942544/1033173712");
AdRequest request = new AdRequest.Builder().build();
mInterstitial.loadAd(request);
}
public void addzeigen(){
if(mInterstitial.isLoaded()){
mInterstitial.show();
addladen();
}
}
I looked at the StackTrace to find the reason for the issue and this two lines indicated that the interstitial is not yet instantiated (returning a null value).
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.google.android.gms.ads.InterstitialAd.isLoaded()' on a null object reference
Because of that I added a null checker in my addladen() method to prevent a crash.
public void addzeigen(){
if(mInterstitial != null && mInterstitial.isLoaded()){
mInterstitial.show();
addladen();
}
}
But why is my Interstitial object null? Did I not instantiate it in the right way in my addladen() method? I already searched for a long time in the internet but found no solution and asked the AdMob support but they said I should ask here because it is an implementation problem. How do I have to modify my code to make sure that my Interstitial is not null? I would be really happy if you could help me.

create application to when open app, add immidiate load
public class MyApplication extends Application {
public InterstitialAd mInterstitialAd;
private AppOpenManager appOpenAdManager;
private Activity currentActivity;
private int check = 0;
public AdLoader adLoader;
#Override
public void onCreate() {
super.onCreate();
MobileAds.initialize(
this,
new OnInitializationCompleteListener() {
#Override
public void onInitializationComplete(
#NonNull InitializationStatus initializationStatus) {
}
});
appOpenAdManager = new AppOpenManager();
loadInterstitial(AD_UNIT_ID_INTERSTITIAL_ADS);
}
public void loadInterstitial(String id) {
AdRequest adRequest = new AdRequest.Builder().build();
InterstitialAd.load(
this,
id,
adRequest,
new InterstitialAdLoadCallback() {
#Override
public void onAdLoaded(#NonNull InterstitialAd interstitialAd) {
mInterstitialAd = interstitialAd;
interstitialAd.setFullScreenContentCallback(
new FullScreenContentCallback() {
#Override
public void onAdDismissedFullScreenContent() {
mInterstitialAd = null;
}
#Override
public void onAdFailedToShowFullScreenContent(AdError adError) {
// Called when fullscreen content failed to show.
// Make sure to set your reference to null so you don't
// show it a second time.
mInterstitialAd = null;
}
#Override
public void onAdShowedFullScreenContent() {
}
});
}
#Override
public void onAdFailedToLoad(#NonNull LoadAdError loadAdError) {
mInterstitialAd = null;
}
});
}
}
then you create class check exception happen when load and open app
public class LoadInterstitialAds {
private static LoadInterstitialAds instance;
private InterstitialAd mInterstitialAd;
public static LoadInterstitialAds getInstance() {
if (instance == null) {
return new LoadInterstitialAds();
}
return instance;
}
public void openAdsThenOpenActivity(Activity activity, InterstitialAdsListener listener) {
Application application = activity.getApplication();
if (application instanceof MyApplication) {
LoadInterstitialAds.this.mInterstitialAd = ((MyApplication) application).mInterstitialAd;
if (LoadInterstitialAds.this.mInterstitialAd != null) {
LoadInterstitialAds.this.mInterstitialAd.show(activity);
LoadInterstitialAds.this.mInterstitialAd.setFullScreenContentCallback(new FullScreenContentCallback() {
#Override
public void onAdDismissedFullScreenContent() {
listener.onStartActivity();
((MyApplication) application).loadInterstitial(AD_UNIT_ID_INTERSTITIAL_ADS);
}
#Override
public void onAdFailedToShowFullScreenContent(#NonNull AdError adError) {
((MyApplication) application).loadInterstitial(AD_UNIT_ID_INTERSTITIAL_ADS);
listener.onStartActivity();
}
});
} else {
((MyApplication) application).loadInterstitial(AD_UNIT_ID_INTERSTITIAL_ADS);
listener.onStartActivity();
}
}
}
}
create interface listener tothe convenience of calling
public interface InterstitialAdsListener {
void onStartActivity();
}
and finally, run this code at any where you want to show
LoadInterstitialAds loadInterstitialAds = LoadInterstitialAds.getInstance();
loadInterstitialAds.openAdsThenOpenActivity(getActivity(), ()->{
// your process
});

Terminating timer in android

I am a begineer in android app development, i have stoptimertask function in my mainactivity, and a button stop in another activity. What i want to do is when i press this stop button from my 2nd activity(which is maps.class), i want the stoptimertask to stop i.e. stop the tasks. However the app crashes.
Here is my code of mainactivity.java
public class MainActivity extends FragmentActivity{
protected static final int CONTACT_PICKER_RESULT = 0;
int count=0;
Timer timer;
TimerTask timerTask;
final Handler handler = new Handler();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
sendBtn.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
startTimer();
sendSMSMessage();
Intent toAnotherActivity = new Intent(MainActivity.this, maps.class);
startActivityForResult(toAnotherActivity, 0);
}
});
}
public void startTimer() {
timer = new Timer();
initializeTimerTask();
if(radioBtnten.isChecked()==true)
timer.schedule(timerTask, 5000, 10000);
// if(radioBtn2.isSelected()==true)
else if(radioBtnone.isChecked()==true)
timer.schedule(timerTask, 5000, 1000);
}
public void initializeTimerTask() {
timerTask = new TimerTask() {
public void run() {
handler.post(new Runnable() {
public void run() {
Toast.makeText(getApplicationContext(), "your message has been sent, the message(s) sent are:-"+count++,Toast.LENGTH_LONG).show();
sendSMSMessage();
}
});
}
};
}
public void stoptimertask(View v)
{
//stop the timer, if it's not already null
Toast.makeText(getApplicationContext(), "Stop button pressed",Toast.LENGTH_LONG).show();
if (timer != null)
{
timer.cancel();
timer = null;
count = 0;
}
MainActivity.this.finish();
}
}
Here is the maps.java(2nd activity)
public class maps extends FragmentActivity implements LocationListener {
MainActivity call=new MainActivity();
GoogleMap googleMap;
Button stop;
Timer timer;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//show error dialog if GoolglePlayServices not available
if (!isGooglePlayServicesAvailable()) {
finish();
}
setContentView(R.layout.maps);
stop = (Button)findViewById(R.id.stop);
stop.setOnClickListener(new View.OnClickListener()
{
public void onClick(View aView)
{
Intent toAnotherActivity = new Intent(aView.getContext(), MainActivity.class);
startActivityForResult(toAnotherActivity, 0);
Toast.makeText(getApplicationContext(), "Stop button pressed",Toast.LENGTH_LONG).show();
maps.this.finish();
call.stoptimertask(aView);
}
});
here is the logcat
FATAL EXCEPTION: main
Process: com.example.textmessage, PID: 19869
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:105)
at com.example.textmessage.MainActivity.stoptimertask(MainActivity.java:167)
at com.example.textmessage.maps$1.onClick(maps.java:49)
at android.view.View.performClick(View.java:4756)
at android.view.View$PerformClick.run(View.java:19749)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Best use for this kind of scenario is Singleton pattern.
MainActivity.java
public class MainActivity extends FragmentActivity
{
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initializeTools();
// Find reference of "sendBtn" with "findViewById" or other stuff
sendBtn.setOnClickListener(
new View.OnClickListener()
{
#Override
public void onClick(View v)
{
startTimer();
}
});
// Rest of your code
}
private void initializeTools()
{
// Give context to Timers instance
Timers.getInstance().giveContext(this);
}
private void startTimer()
{
// Starts the timer when you click on "sendBtn"
Timers.getInstance().startTimers();
}
}
Timers.java
public class Timers
{
private final ScheduledExecutorService scheduledExecutorService;
private final Runnable myTask;
private ScheduledFuture<?> futureTask;
private int count = 0;
private Context _context;
private static volatile Timers _timers;
private Timers()
{
super();
// Your "futureTask manager"
scheduledExecutorService = Executors.newScheduledThreadPool(5);
// Good use is to instanciate task since it won't change on runtime
myTask = new Runnable()
{
#Override
public void run()
{
// Your code to run after the delay has expired
Toast.makeText(_context, "your message has been sent, the message(s) sent are:-" + count++, Toast.LENGTH_LONG).show();
// Same as the whole example, you should use the Singleton pattern to handle communications thanks to the Singleton class "Communicator"
Communicator.getInstance().sendSMSMessage();
}
};
}
// Allow only one instance of the class running. Anyone can get reference of the class with the static function Timers.getInstance();
public static Timers getInstance()
{
if (Timers._timers == null)
{
synchronized (Timers.class)
{
if (Timers._timers == null)
{
Timers._timers = new Timers();
}
}
}
return Timers._timers;
}
// For Toasts and other useful stuff
public void giveContext(Context context)
{
this._context = context;
}
// Stop the timer
public void stopTimer()
{
if (futureTask != null)
{
futureTask.cancel(true);
}
}
// Starts the task to happen in 10 seconds
public void startTimers()
{
futureTask = scheduledExecutorService.schedule(myTask, 10, TimeUnit.SECONDS);
}
}
And inside any class of your application, use Timers.getInstance().stopTimer(); to stop the timer and Timers.getInstance().startTimer(); to start it again.

Did you try?
mTimer = new Runnable() {
#Override
public void run() {
**return;**

Dialog fire non-static methods in main activity

I have a dialog that adds items to a listview, and when an item is added I need to reset the list adapter (because if not things get weird).
I read here that I can create an event listener and listen to it in the main activity. I tried doing so but it gives me errors.
AddMovieDialog.java:
public class AddMovieDialog extends DialogFragment {
private OnFinishListener onFinishListener;
#Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
builder.setView(inflater.inflate(R.layout.add_movie_dialog, null))
.setTitle("Add a movie")
.setPositiveButton("Add", new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
// stuff
if (!movieName.isEmpty()) {
// stuff
if (AddMovieDialog.this.onFinishListener != null)
AddMovieDialog.this.onFinisheListener.finish();
}
}
});
// Create the AlertDialog object and return it
return builder.create();
}
public void setOnFinishListener(OnFinishListener listener) {
this.onFinishListener = listener;
}
public interface OnFinishListener {
void finish();
}
}
In the MainActivity:
AddMovieDialog addMovieDialog = new AddMovieDialog();
addMovieDialog.setOnFinishListener(new OnFinishListener() {
public void finish() {
}
});
But it gives me a compilation error: "The method setOnFinishListener(new OnFinishListener(){}) is undefined for the type AddMovieDialog"

You need to call a method which is non-static using the object. You can't call it using just the class name.
Change to this
AddMovieDialog addMovieDialog = new AddMovieDialog();
addMovieDialog .setOnFinishListener(new OnFinishListener() {
public void finish() {
}
});
Also shouldn't
if (AddMovieDialog.this.onCloseListener != null)
AddMovieDialog.this.onCloseListener.finish();
be
if (AddMovieDialog.this.onFinishListener != null)
AddMovieDialog.this.onFinishListener.finish();
EDIT
Seem your import statement in MainActivity is wrong. It should be something like com.yourpackagename.AddMovieDialog.OnFinishListener

To execute AsyncTask from onClick(DialogInterface dialog)

Currently I have two java class, 1 with AsyncTask and another one is with extend DialogFragment.
I would like to call CreateGroupTask(AsyncTask) in ChooseAddContact java class.
I have tried several recommended ways to execute the AsyncTask java but all failed.
Any recommendation or solution to that?
public class ChooseAddContact extends DialogFragment {
#Override
public Dialog onCreateDialog(Bundle savedInstanceState) {
AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
builder.setTitle(R.string.pick_add)
.setItems(R.array.contact_array, new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
if (which == 0){
AddContactDialog dialog2 = new AddContactDialog();
dialog2.show(getFragmentManager(), "AddContactDialog");
} else if (which == 1){
**How should I CALL it here??
//new CreateGroupTask().execute();
//makegroup = new CreateGroupTask();
//makegroup.execute();
}**
}
});
return builder.create(); }
}
And
public class CreateGroupTask extends AsyncTask<Void, Void, String> {
private Context mContext;
private ProgressDialog pd;
public CreateGroupTask() {
}
public CreateGroupTask(Context mContext) {
super();
this.mContext = mContext;
}
#Override
protected void onPreExecute() {
pd = ProgressDialog.show(mContext, null, "Creating group...");
}
#Override
protected String doInBackground(Void... params) {
String chatId = ServerUtilities.create();
if (chatId == null) return null;
try {
...
} catch (SQLException sqle) {}
return chatId;
}
#Override
protected void onCancelled() {
pd.dismiss();
}
#Override
protected void onPostExecute(String result) {
pd.dismiss();
if (result != null) {
Toast.makeText(mContext, "Group created " + result, Toast.LENGTH_LONG).show();
} else {
Toast.makeText(mContext, "Group creation failed. Please retry later.", Toast.LENGTH_LONG).show();
}
}
}

In Dialog Fragment
To Start the async task you need context. In dialog Fragment you can get the context by calling getActivity(); or else you can get the context or activity reference in onAttach() lifecycle method of dialog fragment. Already parametrized constructor is there better to remove zero parametrized constructor.
new CreateGroupTask(getActivity()).execute();
(OR)
private Activity activity;
onAttach(Activity activity){
this.activity=activity;
//store this activity reference
}
//Then Call
new CreateGroupTask(activity).execute();

You can create an object of the class CreateGroupTask and execute wherever you want.
CreateGroupTask createGroupTask = new CreateGroupTask();
then
` if (which == 0){ AddContactDialog dialog2 = new AddContactDialog();
dialog2.show(getFragmentManager(), "AddContactDialog");
} else if (which == 1){
createGroupTask.execute();
// Or new CreateGroupTask().execute();
}`
try this and tell us if any error

What you are missing in AsyncTask is Context. you are using mContext in below code
pd = ProgressDialog.show(mContext, null, "Creating group...");
but you are not initializing that object in default constructor.
Use getActivity while calling task
new CreateGroupTask(getActivity()).execute();
Also remove following constructor from your code
public CreateGroupTask() {
}